The following is a tabulation of some prior art that presently appears relevant.    Gjunter V. E, Patent Date Nov. 16, 1999 U.S. Pat. No. 5,986,169    Mabe, J. H., Calkins F. T. et al, Patent Date Jun. 29, 2007, U.S. Pat. No. 7,878,459    Moigenier, P. Chenut G., Patent Date 5 Nov. 2, 1999, U.S. Pat. No. 5,975,468    Jacot et al, Patent Date Aug. 10, 2002 U.S. Pat. No. 4,798,051    Jacot et al, Patent Date Feb. 27, 1998, U.S. Pat. No. 6,065,934    Swenson, S. Patent Date Feb. 7, 1991, U.S. Pat. No. 5,127,228    Bansiddhi A., Dunand D. C., J. Mat. Res., 24 (2009) 2107-2117    Bansiddhi A., Dunand D. C. Intermetallics, 15 (2007) 1612-1622    Wen, C. E., Xiong J. Y., Li Y C, and Hodgson P D, Phys. Scr., T139, (2010) 1-7    Qidwai M. A., et al, Int. J. Solids Structures, 38, (2001), 8633-8671
Shape Memory Effect Actuators (SMAs), composed in particular of Nickel Titanium (TiNi) or Nitinol and its tertiary alloys (TiNi—Cu, Nb, Hf), have had limited applicability in large force actuator applications. There are several reasons for this, such as cost, difficulty in attaching the SMA element and finally slow cycle times. The latter is due to the Shape Memory Effect being a first-order phase transformation between a low temperature martensitic phase and a high temperature austenitic phase, and so actuation is done by heating and cooling a Nitinol shape. For most small force applications of less than 100 gms, the high resistivity of Nitinol as a drawn wire means that joule heating can be used to heat the wire provided that the current draw is practical. Cooling is done passively, either by cooling in gas or a fluid. This has limited the applications to small diameter wires. Electrical current requirements are generally less than 1 Ampere and consequently heating and cooling times are reasonably quick, taking several seconds for 0.003″ diameter wire and up to 30 to 60s for 0.020″ diameter wire.
As the required actuation force increases, so does the cross-sectional area of the TiNi SMA element, consequently the cyclic response times of the actuators can be in the order of minutes. Heating must be done indirectly, either by wrapping heater wire around the SMA article, heating tape or other hot element, and this dramatically increases the response time of the actuator. For example, a 0.060″ thin walled tube will take several minutes to heat and significantly more time to cool, reducing their applicability.
To generate large actuation forces with a fast response time, there have been attempts to use bundles of thin TiNi wires to generate the force but keep the actuation cycle times small. (The most common form of TiNi or other SME alloy is as drawn wire that is commercially available from a variety of suppliers.) This approach has been limited in several ways:    1. TiNi wires are very difficult to bond either by adhesives, welding or mechanically, such as by crimping, and so as the number of thin wires increases as the actuation forces increase, the complexity and cost increases and the overall reliability of the joint decreases.    2. TiNi as a material is expensive, and so these bundled actuators become increasingly expensive as the number of wires increases.    3. The mechanical connections to the structure that is to be actuated can be problematic, and under load, be prone to failure.    4. TiNi and its tertiary alloys are difficult to machine, with Electro-Discharge Machining (EDM) and grinding being the two methods that are used. Both techniques are expensive, and so connectors made by machining are expensive.
By fashioning a TiNi actuator article as a open-celled foam, so that heating and cooling fluids can flow easily through the material, and yet the material can be large, the problem of achieving large actuation forces but with a fast response time is elegantly solved.
The manufacture and physical properties of porous shape memory alloys have been described in the literature for over 20 years, starting with Hey and Jardine who described a closed cell TiNi solid made by sintering of Ni and Ti powders. Gjunter in U.S. Pat. No. 5,986,169 described a method of making an open-celled porous TiNi using the technique of High Temperature Self Propagating Synthesis (SHS), which makes TiNi shape memory alloys with a pore size on the order of several hundred microns and foam strut sizes on the order of 100 to 400 microns in diameter. In recent years, there have been many other techniques to manufacture porous TiNi, which are documented by Wen et al. and Bansiddhi. In these expositions, the notion that these foams displayed the Shape Memory Effect is discussed although there has not been any demonstration of the SME nor any attempt to discuss cycle rates.
The SHS technique allows for net-shape TiNi foams to be made. A mold is made that will contain Ni and Ti powders, and Ti and Ni powders are mixed to the desired stiochiometric ratio to allow for the final foam product to exhibit the shape memory effect, typically this is a 50 atomic percent Ti, 50 atomic percent ratio. Additives of tertiary alloys such as Cu, Nb, and others can be added to change the physical characteristics of the shape memory effect.
As described by Gjunter and others, once the powders are inserted into the mold, the powders are pressed to a pressure of 400 to 800 psi and then heated to a desired pre-heat temperature in either an inert gas or vacuum. Once the preheat condition is established, the SHS process is established by a means of locally melting the powder in one location, such as by using a Tungsten filament at 2000° C., and the enthalpy of the reaction is such that it continues to heat surrounding powder to melting, thus generating a self-propagating reaction that consumes the powder. The end physical form of the material is an open-celled foam, as seen in FIG. 1. By varying the process conditions and initial powders, open-celled foams with a strut diameter of several hundred microns can be realized. Open-celled TiNi foams are available in a variety of forms from Shape Change Technologies LLC, Thousand Oaks Calif.
As the SHS is a net-shape process, the molds can be designed to fashion open-celled foam SME actuators that can be designed to supply large actuation forces and strokes that are not available to conventional SME wire or tube actuators. Of particular interest is that the form of the actuator can have ends that are fashioned to be inserted into a complementary mating structure, making structural attachments facile.
Of particular significance, with the small strut diameter, the shape memory effect can be significantly more rapid in foams than in a solid material of comparable actuation force. By utilizing hot and cold fluids or gases under pressure, these fluids and gases can be forced through the open-celled foam which allows for rapid heat transfer and very fast cycle times between hot and cold shapes.
Shape Change Technologies manufactured net-shape TiNi foam torque tubes with rapid Shape Memory Effect actuation. The increase in response time on cooling and cycling times is about an order of magnitude better than non-porous TiNi torque tubes with a cost that is projected to also be an order of magnitude better with batch production.
Of particular interest is that the net-shape nature of the SHS process allows for an elegant solution to the problem of connection of a SMA article that can generate large forces onto an external structure, as well as reducing the need for machining of the article. For example, SMA Foam TiNi torque tubes with integrated hexagonal ends can be made in one step using the SHS process, making attachment simple to an external structure, simply by fashioning the SHS mold so that the hexagonal ends have a tight fit into a hexagonal socket that is mechanically attached to the external structure.
The SMA article can have a variety of forms that can be tailored to match the desired external actuation constraints of available volume, required stroke, connections etc. As such, SMA foam articles can be in the form of tube or solid bars with ends that can be square, hexagonal or more complex geometries if the actuation is accomplished by twisting of the torsional bar or rod. SMA flat sheets with integrated holes can be fashioned if the actuation is required to be a deflection outwards. Finally, the SMA article can be in the form of a helix for large strokes, or in the form of spheres, cylinders, beveled springs and a plethora of other geometries.
Most SMA actuator applications use electrical heating of thin SMA wires as this is a simple and inexpensive means to interface the actuation to a control circuit. Cooling is typically passive, with the heat dissipated in air. For porous foams, a number of techniques can be employed to heat and cool the material, including utilizing the flow of hot and cold fluids, such as gas or liquids, through the pores. Electrical heating by directly heating the SMA article by current can also be performed, although none of the benefits of the heat transfer using hot and cold fluids or gases is realized.
From experimentation, the following has been demonstrated:    1. The Shape Memory Effect was observed and quantified in torsional TiNi foam actuators.    2. The total response time of the effect using hot and cold water is rapid, <5s for a 0.625″ OD, 0.375″ ID torque tube, a dramatic decrease in cycling time.    3. The initial response time of the torque tube when hot water was added was very fast, certainly less than 1/16th second, opening up the possibility of having a sub- 1 Hz bandwidth for a torque tube actuator system.    4. A net-shape torque rod with integrated hexagonal ends has been made. The tube was able to respond rapidly to 83 in-lb of torque, raising it by 1.6° before failing.